(1) Field of the Invention
The invention relates to a transmission system for the transmission of data pulses from a transmitter to a receiver through a transmission medium, wherein the transmitter comprises a pulse source connected to the transmission medium for applying n-valent pulses to the transmission medium, which pulses, when passing through the transmission medium, give rise to the occurrence of intersymbol interference, these pulses furthermore being affected by noise, wherein the receiver comprises a pulse repeater, connected to the transmission medium, with quantized feedback, comprising a forward filter connected between the transmission medium and an input of a difference former, a pulse regenerator connected to the output of the difference former and a clock pulse source, and comprising a feedback filter connected between the output of the pulse regenerator and a second input of the difference former, wherein means are provided between the pulse source and the transmission medium, in the transmission medium itself and between the transmission medium and the forward filter such that a transmission channel having a uniform transmission characteristic is present between the pulse source and the forward filter, which transmission channel may be limited between a lower cut-off frequency differing from zero and a higher cut-off frequency differing from infinity.
Such transmission systems with quantized feedback are particularly suitable for use in situations wherein the transmission channel suppresses certain signal frequencies owing to band limitation. A relevant example is the blocking of d.c. current by transformers in cable links. In such systems the d.c. current component of the transmitted data pulses can be restored in the receiver by quantized feedback. This enables the transmission of pulses without modulation of a carrier, that is to say in baseband.
Furthermore it has been shown in the prior art that optimal non-linear equalization of a transmission path furnishes a higher signal-to-noise ratio and a smaller intersymbol interference than optimal linear equalization. A field in which transmission systems with quantized feedback is used is the transmission through coaxial cables which hitherto predominantly use linear equalization in combination with redundancy codes which suppress the d.c. current component. A special form thereof are the so-called hybrid systems in which a plurality of analog line amplifiers are included between two consecutive pulse repeaters.
(2) Description of the Prior Art
The book Data Transmission by W. R. Bennett and J. R. Davey, published by McGraw-Hill, 1965, pp. 274-275, describes a transmission system in which the transmission channel has a high-pass character. In the receiver the low-frequency signal components are corrected by quantized feedback, utilizing a second order lowpass filter with discrete components.
The article by A. C. Salazar, published in Bell System Technical Journal, March 1974, pp. 503-523 discloses a design of the transmitter and receiver filters in the form of finite non-recursive discrete-time filters. These filters must counteract the ambient noise and the precursors of the pulses. The limiting factor in the design is the quantity of intersymbol interference--caused by the postcursors of the pulses--which must be eliminated by the quantized feedback. As described in the article, one single erroneous decision in the receiver may lead to a burst of errors if the sizes of the precursors are large.